Hair is often subjected to a wide variety of insults that can cause weakening and damage. These include shampooing, rinsing, drying, heating, combing, styling, perming, colouring, exposure to the elements etc. Thus the hair is often in a dry, rough, lusterless or frizzy condition due to abrasion of the hair surface and removal of the hair's natural oils and other natural conditioning and moisturizing components.
A variety of approaches have been developed to alleviate these conditions. These include the use of ultra mild shampoo compositions, the use of hair conditioning shampoos which attempt to both cleanse and condition the hair from a single product and the use of hair conditioning formulations such as rinse-off, leave-on or heat activated products. See, for example, EP-A-181,773, WO-A-97/35542, WO-A-97/35545, WO-A-97/35546, all of which describe conditioning shampoo compositions.
It has been suggested that amino acids may be used in hair care compositions for providing a variety of benefits. For example, U.S. Pat. No. 4,201,235 describes compositions comprising various amino acids that enhance the luster and softness of the hair. JP-A-57/109711 describes compositions comprising amino acids which improve the combing properties of the hair. DE-A-1,617,477 describes hair tonic with guaranteed hair growth comprising certain amino acids. JP-A-59/078114 describes compositions for imparting a smooth feeling to the hair containing amine oxide compounds and amino acids having an isoelectric point of 7 or lower.
It has been found that the levels of certain amino acids are reduced in damaged hair in comparison with undamaged hair. In particular, it has been found that cysteine, tyrosine, lysine, histidine, methionine and tryptophan are at reduced levels in damaged hair. Surprisingly, it has been found that compositions comprising a tyrosine compound and at least one other amino acid compound selected from tryptophan compounds, histidine compounds and lysine compounds can improve the strength and condition of the hair.